I realise that this is an ancient post... but heck... maybe someone else has the same question...
And the answers are:
1) A loooong time!
Whenever I do a format, Avast is the first program I install after the OS!
I leave it to itself and come back around an hour later (dunno if it actually takes a
full hour though).
Then I hook up to the net and download the updates and continue installing whatever program I need to install.
2) I think it does a boot scan only once... As long as you complete a boot scan at least once, it shouldn't bother you for another.
Unless of course you have detected virii and Avast thinks it prudent to do another boot scan, you shouldn't be asked every time for a boot scan.
3) I never knew it had this feature???
I love Avast